A physicist is claiming that solar sailing -- the idea of using sunlight to blow spacecraft across the solar
system -- is at odds with the laws of thermal physics.

But Thomas Gold from Cornell University in New York says the proponents of solar sailing have forgotten about
thermodynamics, the branch of physics governing heat transfer.

Gold writes, The radiation pressure exerted by incoherent light on diverse surfaces is examined. The thermodynamic
rule, first given by Carnot in 1824, describes the limitation to the amount of free energy that can be obtained from a source
of thermal energy, and he gave the compelling reason for this rule, that if more free energy than he had prescribed could
ever be extracted, then a heat pump could use that free energy and re-create all the heat energy that had been consumed. A
perpetual motion machine could then be constructed. Now, 179 years later, it is proposed to fly a spacecraft that is expected
to gain velocity from the radiation pressure the sunlight is expected to exert on solar sails, panels of thin plastic sheets,
mirror surfaced on the side facing the sun. However a detailed examination of this proposal shows it to be in direct conflict
with Carnot's rule, and no such pressure can be expected. Either Carnot's accepted rule is in error, or the solar
sail proposal will not work at all.